Congestive heart failure is a condition in which a heart is unable to circulate enough blood to satisfy the metabolic demands of the body's various tissues and organs. People suffering from congestive heart failure may experience shortness of breath and fatigue. Such people may also exhibit fluid retention, referred to as edema. It is estimated that more than twenty-two million people suffer from congestive heart failure.
There exist many underlying causes of congestive heart failure. For example, congestive heart failure may be caused by a past heart attack, narrowed coronary arteries that restrict the supply of blood to the heart, various heart valve diseases, or congenital heart defects, to name a few. Those suffering from congestive heart failure may exhibit an arrhythmia that exacerbates the patient's heart failure. For example, a patient may exhibit left bundle branch block, a condition in which transmission of an electrical signal to the left ventricle is delayed. Ordinarily, electrical pulses propagate through the heart, causing the various cardiac muscle cells to contract when excited by the pulses. Usually, the cycle of electrical excitation of the heart is initiated by the sinoatrial node. An electrical impulse is generated by the sinoatrial node, and propagates from the sinoatrial node to the right and left atria. For normal propagation of the electrical impulse, the right and left atria contract at substantially the same time. Contraction of the atria force blood from the right and left atria into the right and left ventricles, respectively. Eventually, the electrical impulse reaches the atrioventricular node. From the atrioventricular node, the electrical impulse is carried along right and left bundle branch fibers to a network of fast-conducting Purkinje fibers that extend throughout most of the endocardial surface of the ventricles. The ventricles, when excited by the electrical impulse, normally contract at substantially the same time, causing the blood therein to exit and travel to either the lungs or the peripheral arterial system. In a heart suffering from left bundle branch block, the contraction of the left ventricle is retarded relative to contraction of the right ventricle. This results in an uncoordinated and inefficient heart contraction. Other conduction abnormalities may results from congestive heart failure (e.g., right bundle branch block).
Recently, implantable devices similar to traditional pacemakers or cardioverters (with pacing functionality) have been developed to address the problem of asynchronous left and right chamber contraction in congestive heart failure patients. Typically, these devices deliver electrical impulses to both ventricles of the heart timed to ensure that they contract together, thereby rectifying the asynchrony. Other pacing schemes to resynchronize a heart's contraction also exist. Such resynchronization pacing schemes are referred to generally as “cardiac resynchronization therapy” (CRT). (Cardiac rhythm management devices employing CRT may be referred to as “CRT devices.”) Accordingly, a growing number of patients suffering from congestive heart failure use implantable CRT devices.